1. Field of the Invention
The present invention relates to a covering composition for optical fiber with superior elastic modulus and heat stability as well as a covered optical fiber using the same.
2. Description of the Related Art
In the advanced information society, the use of an optical communication system using glass fiber as information transmission means is rapidly beginning, since glass fiber has such advantages as less optical transmission loss and good flexibility as well as reduced weight. Generally, a covered optical fiber comprising a primer covering layer composed of a primary layer and a secondary layer, an ink covering layer and a matrix covering layer sequentially laminated on the surface, is used as a glass fiber for optical transmission. A photo-curing covering composition is known as covering material.
Recently, with the increase in the construction of optical fiber, thinning of the covered layer has been progressed in order to obtain high density mounting and to reduce the cost. According to such the circumstance, it is required for the covering material to have high elastic modulus for maintaining sufficient physical properties with a thin film.
Japanese Patent Publication No. 6-74307-B discloses a covering composition for optical fiber that provides a cured material with a specific Young's modulus at ordinary temperature as well as low temperature-dependency of Young's modulus. The composition comprises (a) a polymer obtained by adding a diol having molecular chain of a specific structure to a diisocyanate compound, and then reacting a reaction product thus obtained with (meth)acrylic compound having a hydroxyl group, (b) a monomer comprising a bridge alicyclic hydrocarbon compound having two or more ethylenic unsaturated groups and two or more aliphatic rings in a molecule, and (c) a polymerization initiator.
Japanese Patent Publication No. 2525177-B discloses a covering composition for optical fiber that provides cured material that has low temperature-dependency of Young's modulus, superior elongation-after-fracture-properties and reduces a transmission loss of the optical fiber. The composition comprises (A) urethane di(meth)acrylate having a number average molecular weight of 1000 to 15000 and (B) urethane di(meth)acrylate having a number average molecular weight of 800 or less.
However, such a problem occurs that a covered film formed of conventional covering materials suffers great film deformation accompanying temperature change, resulting in deterioration of optical transmission property of the optical fiber. Further, since a fiber covered with the film with a larger blend ratio of the covering material has reduced flexibility, it is difficult to well balance heat stability and flexibility.